This invention relates to an antenna switch circuit, and more particularly to an antenna switch circuit having an improved characteristic in a microwave band.
In a portable telephone system which employs a TDD (Time Division Duplex) system, transmission and reception are performed separately from each other in time by time division processing. However, a portable telephone system which has been recently started in Japan employs a CDMA (Code Division Multiplex Access) system, in which transmission and reception are performed simultaneously.
Therefore, occurrence of cross modulation causes a problem with a telephone terminal of the CDMA system. Such cross modulation is caused by mixture of part of a transmission signal of a terminal with an original reception signal as the transmission signal is received by a reception circuit of the terminal itself, and makes a disturbing component to the reception signal. Occurrence of cross modulation is described below.
Where an antenna switch circuit uses an FET of the junction type which includes, for example, a gallium-arsenic substrate, the FET which is a non-linear element has a transfer function which can be represented as an equation (1) of FIG. 6. It is to be noted that, in the expression (1), y is the output voltage, x the input voltage, and H1, H2, H3, . . . are the coefficients of non-linear terms. The coefficients H1, H2, H3, . . . are called nuclei of a Volterra series and are used where the non-linear element includes a capacitor or the like and the non-linearity of it matters.
Where it is considered that the input voltage x is a sum signal of a continuous wave signal as a reception signal and an amplitude modulated wave signal as a disturbing wave signal, the input voltage x(t) can be represented as an expression (2) of FIG. 6. It is to be noted that, in the expression (2), V is the amplitude, b1 the modulation degree of the AM signal, .omega.1 the carrier frequency (angular frequency) of the disturbing wave signal, .omega.2 the carrier frequency (angular frequency) of the reception signal, .omega.m an amplitude modulation component of the disturbing wave signal, and CC a complex conjugate.
Since the signal voltage x(t) is supplied to the FET which is a non-linear element, by substituting the expression (2) into the expression (1), the output voltage y(t) given by an expression (3) of FIG. 6 is obtained. In the expression (3), .theta.1 and .theta.2 are the phases of the nuclei H1 and H2 of the Volterra series.
This represents that, if two signals having conditions given by the expression (2) of FIG. 6 are supplied to a non-linear element represented by the expression (1) of FIG. 6, then such an AM component as given by the expression (3) of FIG. 6 is produced and the amplitude of the original reception signal is influenced by the amplitude modulation component .omega.m of the disturbing wave signal. The amplitude modulation component .omega.m of the disturbing wave signal cannot be removed by means of a filter or a like means any more. This is a cross modulation component. Then, the cross modulation component makes a noise signal to the reception signal and causes deterioration of the reception sensitivity.
By the way, where an FET is used to form a switch circuit, such a connection scheme, for example, as shown in FIG. 4 is employed. Referring to FIG. 4, the switch circuit shown includes an FET (Q1) in the form of, for example, an FET of the junction type made of semiconductors of gallium and arsenic. The FET (Q1) is used as a switch. The switch circuit further includes biasing resistors R1 to R4, an input signal voltage source VS, and controlling voltage sources VC1 and VC2. The input signal voltage VS is supplied to the source of the FET (Q1), and an output from the drain of the FET (Q1) is used as an output of the switch circuit. A drain-source resistance RDS of the FET (Q1) depends upon a gate-source voltage VGS and has such a characteristic as, for example, illustrated in FIG. 5. Accordingly, when VGS.gtoreq.VON, the resistance RDS is almost equal to zero and the FET (Q1) exhibits an on state, but when VGS&lt;VON, the resistance RDS is sufficiently high and the FET (Q1) exhibits an off state.
In the circuit of FIG. 4, since VGS=VC2-VC1, the FET (Q1) can be controlled between on and off by varying both or one of the control voltages VC1 and VC2, for example, by varying the control voltage VC2. When the FET (Q1) is on, the input signal voltage VS is extracted at the output side through the FET (Q1).
However, when the FET (Q1) is on, a position of the voltage VDC(=VC2-VC1) shown in FIG. 5 provides an operating point, and the signal voltage VS exhibits a deflection around the voltage VDC. Then, in the case of FIG. 5, the signal voltage VS exhibits a lower voltage at a portion thereof indicated by slanting lines in FIG. 5 than the voltage VON, and therefore, the signal voltage VS suffers from a distortion at the portion thereof indicated by the slanting lines. Further, if a reception signal and a leak signal of a transmission signal are supplied to the switch circuit as described above, then a cross modulation distortion is produced on the output voltage of the switch circuit due to the non-linearity of the characteristic of FIG. 5 at the portion lower than the voltage VON.
In order to prevent production of such a distortion as described above, the voltage VDC should be set high. In this instance, in order to reduce production of a cross modulation distortion by approximately 20 dB in a portable telephone set of the CDMA type, a voltage of approximately 6 V is required.
However, a portable telephone set is driven by a battery, and the voltage of the battery is designed to be as low as possible such as 3 V or less in order to achieve power-saving. Accordingly, employment of the voltage VDC of approximately 6 V is not preferable in terms of the power consumption, and a solution to this problem is demanded.